We will describe methods of creating a dipole magnet using high temperature superconductors as sources of magnetic fields. We take as an example a dipole which is to produce a vertical magnetic field over a total volume of 50×30×1000 mm, as shown in FIG. 1. In the example to, allow space for an electron beam vacuum chamber and room temperature to cold transitions the vertical magnet pole gap should be 60 mm (i.e. the superconductors must be spaced (at least) 60 mm apart). In this example the field is required to be uniform to +/−0.01% and the integrated field is required to be uniform to +/−0.05%). In the example, the minimum flux density for the magnet should be 3 Tesla.
Traditionally a uniform field will be produced using a cos θ winding. This approach uses a variable pitched winding in which the current in each winding is the same. This winding is based on spacing conductors around a cylindrical axis according to the cosine of their position thereby producing a field which varies sinusoidally around the cylinder and results in uniform field in the centre of the cylinder.
Background prior art can be found in: US 2006/0261812 A1; GB 2289343 A; US 2007/0146107 A1; GB 2433600; U.S. Pat. No. 5,479,144; U.S. Pat. No. 5,389,907; U.S. Pat. No. 4,683,434; JP 10-328159; JP 63-281411; and GB 2285313 A.
However an alternative approach would be to use bulk superconductors as the source of the magnetic field. YBCO bulks are able to trap significant amount of magnetic field. Currently the highest trapped fields are claimed by the Japanese (Tomita M and Murakami M “high-temperature superconductor bulk magnets that can trap magnetic fields of over 17 T at 29 K “Nature 421 517-20) with 17.24 Tesla in a 2.65 cm puck at 29 Kelvin.